It is known that natural lysozyme is an enzyme of peptidic origin, widely present in many species of the animal and vegetal world, having in the different species comparable but not identical structure and enzymatic mechanism. The most known lysozyme is that present in the chicken egg (chicken egg-white lysozyme c). Lysozyme is widely distributed in the mammalians body; it is present for example in saliva, in tears, in milk, in leukocites, in cervical mucus. In 1922 Alexander Fleming discovered a substance present in his own nasal mucus able to lysate some bacterial strains. This substance has been later on identified as lysozyme (today better identified as human lysozyme c). In 1963 Jolles and Canfield independently clarified the primary structure of lysozyme purified from chicken egg-white. In 1965 Phillips describes the tridimensional structure of lysozyme, on the basis of the X-rays crystallography. As from its discovery, natural lysozyme (more precisely the chicken egg-white lysozymne c) has been used sometimes in medicine mainly for its anti-bacterial properties, but the retrospective analysis of the published studies has not even today fully clarified its specific mechanism of action and in some cases the experimental results seem to be even contradictory (1,2,3).
Nowdays scientists think that the biological action of natural lysozyme c shall be divided in two different ways: a direct action and an indirect action. The direct action is determined by the degradation of the link (β, 1-->4) between N-acetylmuramic acid and N-acetylglucosamine (4) of peptidoglycanes present in the cell membrane of bacteria, thus determining the lysis of the same (as consequence of this action it is frequently called “muramidase”). Generally Gram positive bacteria are more sensitive to natural lysozyme's action than Gram negative, probably because the latter ones present the external membrane, as further barrier. This direct action of lysozyme is potentially important for its possible therapeutic use in prophylaxis and treatment of many bacterial diseases, alike faringitis, congiuntivitis, otitis, sinusitis, adenitis, uretritis, vaginitis, cistitis, substantially caused by microorganisms of Streptococci family. By contrast, the degradated mucopeptidic fragments of the cell membrane, produced by the direct action of lysozyme on the bacteria, seem to be able to elicit a not well defined process of immunological stimulation (indirect action of lysozyme).
Some authors have also recently described the capacity of chicken egg-white lysozyme c and of its derivatives to stimulate the production of lymphocytes T present in the lymphatic tissue, associated to intestine (GALT), and in spleen (GALT-speen axis).
However in literature some authors have also described the use of other specific lytic peptides for the treatment of many diseases. In some recent patents the specific combination of the administration of Cecropins and lysozyme has been suggested, in order to increase the action of the two compounds. In fact some authors suggest that Cecropins are much more active in lysing the cell, while lysozyme is more active in its complete degradation.
The new chemotherapeutic agents, like Cecropins, Sarcotoxins, Magainins, Lepidopterans, having the same physical properties with an α-elique structure and hydrophobic character, are proteins able to lysate not only the cellular membrane of bacteria (Listeria monocitogenes and Brucella abortus), but also the cell membrane of protozoos (P. Falciparum) and viruses (Parainfluenza and Herpes Simplex), and of eukariotes cells infected with bacteria. By using an electronic microscope it has been discovered that lytic peptides produce large pores into membrane of cells, determining their death. By contrast, healthy mammalian cells are not destroyed by the above lytic peptides, due probably to the presence of citoskeleter, which is in contact with various points of the plasmatic membrane and consequently it permits to maintain the osmotic integrity of the cell.
Moreover also at low concentrations the lytic peptides shall be used as biological response modifiers, in order to stimulate the cells proliferation of the immunological system and of the skin. The above action is probably caused by the production of pores in the plasmatic membrane, determining a flow of ions and of nutritive material into the cells, which stimulates the cells' growth.
Nevertheless, although many publications evidence the positive action of natural lysozyme (mainly chicken egg-white lysozyme c) in the treatment of infections of various origin, the potential use of group c lysozyme in clinics has been very limited until nowadays due to three major factors:
a) human lysozyme c (which is the type present in humans) is extremely difficult to be obtained in sufficiently large amounts for therapeutic applications;
b) other lysozyme c types, which are obtainable from other species, alike bovine and chicken egg-white, may present some negative side-effects (the first one is possible allergic reactions and the second one is the potential risk of contamination of transmissible bovine encephalopathies or TBE), which are considerably limiting their use in medicine;
c) lysozyme types, different from c, originating from other animal species are not suitable for medicinal purposes since they present a very low homology with human lysozyme c.
A table is reported wherein the homology between human lysozyme and lysozymes from other mammalians, however all of natural c type, is indicated:
LYSOZYME ORIGINHOMOLOGY %Egg (white) chicken56.8%Rat76.4%Cow82.3%Rabbit82.3%Horse50.8%
Although bovine lysozyme c shows an homology higher than that of chicken egg-white, the latter one is the most used in the clinical practice due that it is very commonly available in suitable quantities at convenient prices. On the basis of the above introduction there is a considerable interest to further explore new possibilities of use of natural lysozyme c, of its possible derivatives and to produce medicinal preparations, which permit to widen its use to some serious diseases in humans and/or to utilize new and previously unexplored administration routes.
Despite many experimental researches on animals and recent publications on lysozyme c, the medicinal applications of this compound when administered orally remained limited to traditional pathologies, already well known to the experts.
However these studies seem to have achieved very limited and sometimes contrasting results.